It’s easy to miss amid the day to day headlines of global economic implosion, Presidential campaign foibles and Mideast rage, but there is a less conspicuous kind of social upheaval underway that is fast altering both the face of the planet and the way that human beings live. That change is the rapid acceleration of urbanization, as more and more people in every corner of the world put down their farm tools and move from the countryside or the village to the city. In 2008, for the first time in human history, more than half the world’s population was living in towns and cities. And as a new paper published in the Proceedings of the National Academy of Sciences shows, the process of urbanization will only increase in the decades to come — with an enormous impact on biodiversity and potentially on climate change.

As Karen Seto, a professor of the urban environment at Yale and the lead author of the PNAS paper, points out, that the wave of urbanization isn’t just about the migration of people into urban environments, but about the environments themselves becoming bigger to accommodate all those people. Today urban areas — ranging from Times Square to a small town in India — cover perhaps 3 to 5% of global land. But Seto and her co-authors calculate that between now and 2030, urban areas will expand by more than 463,000 sq. mi. (1.2 million sq. km). That’s equal to 20,000 U.S. football fields being paved over every day for the first few decades of this century. By then, a little less than 10% of the planet’s land cover could be urban. “There’s going to be a huge impact on biodiversity hotspots and on carbon emissions in those urban areas,” says Seto.

The bulk of that great urban expansion will be in Asia — where more than 75% of the increase in urban cover is projected to occur — and in Africa, where urban land cover will be 590% above the 2000 level of 16,000 sq. mi. (41,000 sq. km). In China and in India, cities will balloon — especially smaller, second-tier cities like Dalian or Pune that often lack the attention and the funding of megacities like Guangzhou or Shanghai. That’s worrying because much of the urbanization wave is happening with little to no advance planning, amplifying the environmental cost of stuffing hundreds of millions of poor people into half-built metropolitan areas that often lack basic sanitation, waste management or water services. “The growth is really going to be in those medium-sized cities, and that’s where the planning has often been lacking,” says Lucy Hutyra of Boston University, a co-author of the PNAS paper.

Those areas of Asia, Africa and parts of South America that will see urban territory grow most rapidly tend to overlap with biodiversity hotspots, concentrations of exotic plants and animals. Humans are the ultimate invasive species — when they move into new territory, they often displace the wildlife that was already living there. The PNAS researchers estimate that urban expansion will encroach on or displace habitats for 139 amphibian species, 41 mammalian species and 25 bird species that are either critically endangered or endangered. And as land is cleared for those new cities — especially in the densely forested tropics — carbon will be released into the atmosphere as well. “In developing countries, there is a lot of pristine land and hotspots that could be threatened by the process of urbanization,” says Seto.

All of this might seem surprising to people who’ve read this blog for the past couple of years. Counter-intuitive as it might be, we’ve usually presented urbanization as a good thing for the environment — and especially for carbon emissions. In the U.S., it’s residents of cities like New York that tend to have smaller carbon footprints, especially compared to their counterparts in the countryside and the suburbs. Dense urban areas reduce commute distances — saving gas — and allow residents to forgo cars altogether. That density also pushes urban residents to live in smaller homes, which in turn means less energy is needed for heating and cooling living spaces. If urbanization tends to be good for the environment in the U.S. and Europe, why wouldn’t that be the case in the rest of the world?

It’s true that as people in developing nations move from the countryside to the city, the shift may reduce the pressure on land, which could in turn be good for the environment. This is especially so in desperately poor countries like Madagascar, where residents in the countryside slash and burn forests each growing season to clear space for farming. But the real difference is that in developing nations, the move from rural areas to cities often leads to an accompanying increase in income — and that increase in income leads to an increase in the consumption of food and energy, which in turns produces an uptick in carbon emissions. Getting enough to eat and enjoying the safety and comfort of living fully on the grid is certainly a good thing — but it does carry an environmental price. In the U.S. urban dwellers tend to be poorer than their counterparts in the suburbs —though the recent gentrification of top-tier cities like Washington and San Francisco has altered that dynamic — and consume less, especially energy. But that’s not so in the developing nations.

The urbanization wave can’t be stopped — and it shouldn’t be. But the PNAS paper does underscore the importance of managing that transition. Seto notes that around 65% of the urban land core in 2030 has yet to be built. If we do it the right way, we can mitigate urbanization’s impacts on the environment. “There is an enormous opportunity here, and a lot of pressure and responsibility to think about how we urbanize,” says Seto. “The one thing that’s clear is that we can’t build cities the way we have over the last couple of hundred years. The scale of this transition won’t allow that.” We’re headed towards an urban planet no matter what, but whether it becomes a heaven or a hell is up to us.

Excuse me? Urbanization shouldn't be stopped? You are on the fast track to hell and you still support cities? Cities are cities of cultural marxism, degenerate art, autism, air pollution, overcrowding, and especially DISASSOCIATION FROM THE EARTH AND FELLOW MAN AND AND FELLOW WOMAN.

Cities are the brutes that make the decisions, they ask for massive quantities of food and luxuries from across the land. Perhaps in the future those that provide for the brutes should be able to make their voices heard in a fairer way, they live in ways the cities cannot understand, they know the land, they know how much they can provide. Above all, these are the people that understand nature and how much you can take before the Earth will give no more. Technology could greatly improve this relationship.

"In the U.S., it’s residents of cities like New York that tend to have smaller carbon footprints, especially compared to their counterparts in the countryside and the suburbs."

That's because it's people in the countryside and suburbs that are manufacturing things and growing food. Urban citizens don't have a smaller carbon footprint when you take into account their consumption of goods produced elsewhere, and the carbon cost of producing those goods.

A friend told me yesterday that in Mumbai, which is an island, 7000 additional cars enter the city every day. There's little good to a shift towards urbanization. In the developed world I can see the benefits mainly because we've just forgotten how to live close to the land and rural dwellers drive hundreds of miles every week. But everywhere else, I don't know.

The point made about the environment in Madagascar being better off if people urbanize doesn't make sense to me. If those people move to the cities they'd still need to eat... and large-scale farming is probably what would take place. It's already happening in India with the big agriculture companies moving in.

However, if we had a system of government that could actual govern properly, urban living could be much more pleasant.

I've only traveled there on business, but I think I would love living in a well-managed city like Singapore - clean, safe, functional, good education.

Maybe having more cities break away into quasi-independent city-states could help. Smaller units seem easier to properly govern - think Singapore, Taiwan, Switzerland, and Luxembourg. Maybe they could confederate to keep some common standards and trade agreements so they could still benefit from the economies of scale of larger units of government as well.

Be realistic. Cities has changed temp.messurements, not showing correct total development. CO2 emissions by humans is not having impact on climatedevelopment. Urban areas are still very small compared with the ocean areas. There is a blind belief in manmade climate changes, scientists with new findings is not presented in Time / Time bloggs thus the picture drawn by media like Time is wrong. Extreme weather yes, acrtic ice melting yes, but not proved to be manmade!

This is all being driven by over-population. The world needs a 1-child policy and quick. In America the fertility rate of our citizenry 2.1 children per female. So why are our cities expanding? Out of control immigration - both legal and illegal. We are drowning in the developing world's excesses and no one has the political will to do anything about it.

Still no one seems to want to talk about voluntary population reduction... With the author admitting that humans are the worst invasive species, it seems obvious that we should be educating this fact and encouraging the use of more birth control while shunning families like the Duggars

Despite shortcomings, cities also offer solutions to numerous sufferings of rural migrants. For them these are places of opportunities for better income, living and learning. These are hubs of knowledge amp; creativity that precedes inventions and leads civilizations to glory. So, do not be afraid of expanding urban foot prints. We live in an age of hope. The wise and creative people living in cities would ensure that extended urban environment remains more liveable and environment friendly.

I don't like living in Lincoln, NE. It's now too big for me. And, it isn't even to 270,000 yet. I would love to move to a rural area, but can't afford to. Can't understand why anyone would want to live in a city so huge. Lincoln used to be a nice smallish city, until the "City Fathers" decided to push for more people coming in. A large tax base you know!

That said, I do agree that urbanization has benefits in terms of living green. There are many different types of efficiencies that can be gained on a per-person level when you're dealing with high population density. Mass Transit is far more efficient, as is power distribution. Pollution can be cleaned more efficiency as you have millions of peoples' worth of pollution in a couple hundred square mile chunk of land, so you have less area to cover for the same amount of polution controls. In fact, I think almost all of the reasons that urban areas are so polluted right now are also reasons they can become less polluted in the future.

I'd be in favor of voluntary population reduction if it could be applied equally across the globe.

I'd be putting my genes at a competitive disadvantage if I limited myself to 1 or 2 children while the average fertility rate in other parts of the world is closer to 3 children per woman.

The only way I can see a global voluntary population reduction is through either a strong one-world government, which has its obvious problems, or expanding materialism and consumerism to distract people from having as many children. This has its own environmental problems, even if population were to decrease.

Oh, there's nothing like living in a big city. I'm proud to be a resident of London just on the edge of Zone 1. Easy public transport, or if you're feeling leisurely, you can pretty much walk anywhere you want to go, taking in the city as you go. I love the vibrancy, seeing new faces every day, changing trends. I love the free stuff- newspapers, food samples, whatever you can scavenge off the street. I love the second hand stores. I love the history, seeing blue plaques that say "so and so lived here", visiting the British Museum or Highgate Cemetery, learning that building across the road from me used to be a tube station.

I love having choice- I can shop at the family owned Mediterranean supermarket, or the hippy one. In my home town the choice is the rip off corner shop or driving to the big corporate supermarket.

I miss having a garden, but now I'm getting involved with a community one. Living in a mansion block is great because you're up above the pollution and other people's heating keeps you warm. I live in a flat with 2 bedrooms and no living room, though we have space to eat in the kitchen so that makes it okay. I like living in a small space.